1. Field of the Invention
Embodiments of the present invention relate in general to combustors and, more particularly, to fuel-air mixers of lean-premixed combustors for use in low-emission combustion processes.
2. Description of the Related Art
Historically, the extraction of energy from fuels has been carried out in combustors with diffusion-controlled (also referred to as non-premixed) combustion where the reactants are initially separated and reaction occurs only at the interface between the fuel and oxidizer, where mixing and reaction both take place. Examples of such devices include, but are not limited to, aircraft gas turbine engines and aero-derivative gas turbines for applications in power generation, marine propulsion, gas compression, cogeneration, and offshore platform power to name a few. In designing such combustors, engineers are not only challenged with persistent demands to maintain or reduce the overall size of the combustors, to increase the maximum operating temperature, and to increase specific energy release rates, but also with an ever increasing need to reduce the formation of regulated pollutants and their emission into the environment. Examples of the main pollutants of interest include oxides of nitrogen (NOx), carbon monoxide (CO), unburned and partially burned hydrocarbons, and greenhouse gases, such carbon dioxide (CO2). Because of the difficulty in controlling local composition variations in the flow due to the reliance on fluid mechanical mixing while combustion is taking place, peak temperatures associated with localized stoichiometric burning, residence time in regions with elevated temperatures, and oxygen availability, diffusion combustors offer a limited capability to meet current and future emission requirements while maintaining the desired levels of increased performance.
Recently, lean-premixed combustors have been used to further reduce the levels of emission of undesirable pollutants. In these combustors, proper amounts of fuel and oxidizer are well mixed in a mixing chamber or region by use of a fuel-air mixer prior to the occurrence of any significant chemical reaction in the combustor, thus facilitating the control of the above-listed difficulties of diffusion combustors and others known in the art. Conventional fuel-air mixers of premixed burners incorporate sets of inner and outer counter-rotating swirlers disposed generally adjacent an upstream end of a mixing duct for imparting swirl to an air stream. Different ways to inject fuel in such devices are known, including supplying a first fuel to the inner and/or outer annular swirlers, which may include hollow vanes with internal cavities in fluid communication with a fuel manifold in the shroud, and/or injecting a second fuel into the mixing duct via cross jet flows by a plurality of orifices in a center body wall in flow communication with a second fuel plenum. In such devices, high-pressure air from a compressor is injected into the mixing duct through the swirlers to form an intense shear region and fuel is injected into the mixing duct from the outer swirler vane passages and/or the center body orifices so that the high-pressure air and the fuel is mixed before a fuel/air mixture is supplied out the downstream end of the mixing duct into the combustor, ignited, and ignited.
Because of the cross jet flow and localized fuel injection points and the way the swirl is imparted, fuel concentrations in conventional fuel-air mixers are highest near the mixer walls at an exit plane, thus preventing the control of the local variation of fuel concentration at the exit of the mixing duct, particularly when considering the need for combustors capable of operating properly with a wide range of fuels, including, but not limited to, natural gas, hydrogen, and synthesis fuel gases (also known as syngas), which are gases rich in carbon monoxide and hydrogen obtained from gasification processes of coal or other materials. Therefore, the fuel concentration profile delivered to the flame zone may contain unwanted spatial variations, thus minimizing the full effect of premixing on the pollutant formation process as well as possibly affecting the overall flame stability in the combustion zone.
Therefore, a need exist for a fuel-air mixer for use in lean-premixed combustors having enhanced capabilities to control the local variation of fuel concentration at an exit thereof while maintaining control of flow separation and flame holding in the mixing duct. This increased control will permit the development of premixing devices having a reduced length without substantially affecting the overall pressure drop in the device.